Order an Array like another Array in C#

Question

I'm looking for the best algorithm to take array A {0,1,2,3} and make order it like array B {3,1,0,2}. Any ideas?

Answer 1

So if you have two arrays and they hold the same data just in different order then just do this:

A = B

I suspect that is not your situation so I think we need more info.

Answer 2

In the example you gave (an array of numbers), there would be no point in re-ordering A, since you could just use B.

So, presumably these are arrays of objects which you want ordered by one of their properties.

Then, you will need a way to look up items in A based on the property in question (like a hashtable). Then you can iterate B (which is in the desired sequence), and operate on the corresponding element in A.

Answer 3

Both array's contain the same values (or nearly so) but I need to force them to be in the same order. For example, in array A the value "3045" is in index position 4 and in array B it is in index position 1. I want to reorder B so that the index positions of like values are the same as A.

Answer 4

If they are nearly the same then here is some pseudo code:

Make an ArrayList
Copy the contents of the smaller array to the arraylist
for each item I in the larger array
    FInd I in the ArrayList
    Append I to a new array
    Remove I from the arraylist

Answer 5

What you need to do is determine the ordering of B and then apply that ordering to A. One way to accomplish this is to undo the ordering of B and keep track of what happens along the way. Then you can do the reverse to A.

Here's some sketchy C# (sorry, I haven't actually run this)...

Take a copy of B:

List<int> B2 = new List<int>(B);

Now sort it, using a sort function that records the swaps:

List<KeyValuePair<int,int>> swaps = new List<KeyValuePair<int,int>>();
B2.Sort( delegate( int x, int y ) {
   if( x<y ) return -1;
   if( x==y ) return 0;
   // x and y must be transposed, so assume they will be:
   swaps.Add( new KeyValuePair<int,int>(x,y) );
   return 1;
});

Now apply the swaps, in reverse order, to A:

swaps.Reverse();
foreach( KeyValuePair<int,int> x in swaps )
{
   int t = A[x.key];
   A[x.key] = A[x.value];
   A[x.value] = t;
}

Depending how the built-in sort algorithm works, you might need to roll your own. Something nondestructive like a merge sort should give you the correct results.

Answer 6

Could the issue be resolved using a Dictionary so the elements have a relationship that isn't predicated on sort order at all?